The best way to explain how to do this is show you my own example of what @kamlesh_vaghela was suggesting.
Here is a repo for my own app with custom JS/CSS components: https://github.com/thibodux/developer_gadgets
if you look under appserver/static/components you will see some of my custom JS classes/modules. You need to put the JS and CSS files for the menu in this kind of folder structure. Then you need to create another JS file that sits in appserver/static that will initialize and pull in the components. @kamlesh_vaghela referred to this as code.js, and in my app repo it is called load.js (https://github.com/thibodux/developer_gadgets/blob/master/appserver/static/load.js). It doesn't matter what you name the file.
The primary requirement for your dashboard is that it must include this top-level JS file in the dashboard or form XML element like @kamlesh_vaghela showed with <dashboard script="code.js" ...> . You can look at the XML files in my app repo to see how I do the same thing.
Now, I am not making any guarantees that this will actually work with the menu library you want. But this is the approach you use to incorporate such custom things in a Splunk app/xml dashboard.
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